Hard tissues of the anatomy are often subject to damage or degradation, which require repair and reconstruction. In the case of teeth, dental decay is one of the most common type of disorders. It is typically caused by the bacteria which are commonly present in the mouth. Although the mechanism by which bacteria cause decay is not completely understood, it is believed that cariogenic organisms, by using sucrose, and to a lesser extent other sugars, produce polymers which bind the organisms to the tooth surface and acids which cause the demineralization resulting in cavity formation.
The decay of bones and especially teeth is referred to as "caries," and the resulting cavity forms a carious lesion in the tooth. Depending upon the degree of advance state of such decay, it may affect the various parts of a tooth, the principal components being the enamel, the dentin, the pulp, and the cementum.
The enamel of a tooth is the intensely hard calcareous (i.e., calcium based) substance that forms a thin layer which caps or partly covers the teeth of most mammals, including humans and other vertebrates. The enamel is the hardest substance of the animal body and its strength allows a tooth to perform its laborious function over many years of life of most humans. The hardness of the enamel also serves to protect the sensitive living tissue within.
The dentin also comprises calcareous material similar to bone but harder. The dentin is a living tissue comprised of a matrix of minute tubules which enter into the inner cavity of the tooth where the living tissues thereof are housed. The "pulp" of the tooth comprises the living or viable tissues of the tooth which are contained within the pulp cavity. The pulp cavity comprises the pulp chamber, located near the crown of the tooth, and the root canal, which extends down to the very proximal or periapical regions of the tooth. The pulp is comprised of connective tissue, blood vessels which nourish the tooth, and nerves which transmit pain and other signals to the brain. The blood vessels and nerves enter the tooth at the tip or apical section of the root canal. Depending upon the type of tooth, there is significant circulation of living matter within the pulp of the tooth.
The cementum is a thin, fairly hard bone tissue covering the root of the tooth. Surrounding the cementum is the periodontal ligament which serves to mount the tooth in the bony socket or alveolus which is formed in the alveolar bone or jaw bone in which the teeth are set.
Dental decay is most likely to affect the enamel, dentin and/or pulp. Once dental caries is found in the enamel, the typical therapy is to remove it in order to prevent further penetration of the decay into the tooth. Such penetration could spread infection throughout the mouth and the body, and possibly result in loss of the tooth. Such therapy for cavities formed by decay is typically referred to as a "filling." In accordance with this well-known procedure, a dentist or other authorized practitioner drills out the cavity formed by the decaying material and may also form undercuts in order to secure the filling material. The dentist then fills the cavity with a filling material which replaces the portion of the tooth lost to decay. This filling material is placed downward into the tooth from the upper or crown regions of the tooth, and is typically referred to as an "orthograde" filling. The dentist then packs the filling material densely and shapes it appropriately.
Once a carious lesion penetrates the enamel and enters the dentin, the viable tooth processes are affected and the tooth may become painful. At this stage, the infectious organisms have relatively ready access to the pulp. Thus, there is a likelihood of formation of abscesses indicated by swelling, pus and sometimes severe pain. The abscesses are typically formed in the periapical tissues surrounding the apex or apical section of the tooth where it intersects the alveolar bone. At this stage of the disease, the tooth can be treated successfully by root canal therapy. In this process, access to the pulp is achieved by drilling whereupon the dead or decayed pulp is removed by the use of small files. The pulp chamber, including the root canal, is then filled or obturated with an inert filling material in order to prevent, ideally, the return of infectious organisms from the mouth to the living periapical tissues surrounding the root tip.
Root canal therapy is not always completely successful. It is estimated that in about 20-30% of the cases, despite the obturation of the root canal by filling material, infection returns. This is usually caused by the migration of bacteria and other infectious organisms from the mouth along the root canal cavity. The bacteria migrates through the interstices of the canal to the living periapical tissues which remain following the root canal therapy. Furthermore, the root canal system is complex and cannot always be completely cleaned with the present techniques and instruments. Thus, periapical abscesses may again form at the tip of the root canal. In these cases, the interstitial seal between the orthograde filling material and the walls of the root canal is not sufficient to prevent the invasion of infectious matter.
Retreatment of unsuccessful root canal cases is the preferred therapy, and it usually results in a successful outcome. However, when non-surgical attempts prove unsuccessful or are impossible, a surgical method known as "apicoectomy" is available. Under this procedure, the patient is placed under a suitable anesthesia and the gums are surgically cut to expose the infected tooth apex. The apex of the tooth is then removed and the resected root end is prepared. Finally, a "retrograde" filling material is inserted into the exposed cavity at the root tip in order to, hopefully, seal the root canal and prevent the migration of oral bacteria into the living periapical tissues. Clearly, this apicoectomy is a painful and expensive procedure. It could be avoided through the use of cavity-filling materials which form effective seals against the migration of infectious organisms.
Various materials have been used to fill teeth. Amalgam is the most common filling material, but others have also been used.
A tooth filling material must exhibit various qualities. It must exhibit good adherence and adaptability to the tooth walls of the cavity. It must be compatible with the surrounding tissue, and must not cause staining or other adverse effects to the surrounding tooth structure. It must harden to provide structural support for the tooth, including its biting surface. It must be relatively easy to apply in what is sometimes a difficult environment due to blood, moisture and potential access problems. It is also important for diagnostic purposes that a filling material be radiopaque, i.e., exhibit a high absorption of the short wavelength x-ray radiation utilized in dental diagnostics. It should also be sterile or easily sterilized.
When filling tooth cavities, it is important to keep the fluid and bacteria present in the mouth from reentering the cavity and causing further decay. Thus, the chosen filling material must adequately seal the cavity to prevent the migration of such fluid and bacteria into the cavity. As noted above, this sealing ability of the filling material is particularly important when the decay has caused access to the pulp of the tooth.
As with conventional fillings and root canals, various materials have been suggested as retrograde filling materials. An ideal retrograde filling material should have many of the same qualities desired in an orthograde filling material. It should adhere and adapt to the dentinal walls of the root end preparations, should prevent leakage of microorganisms and their byproducts into the periapical tissues and should be biocompatible with the periapical tissues. It should also be insoluble in tissue fluids, dimensionally stable, and unsusceptible to the presence of moisture. Additionally, any antimicrobial qualities are beneficial, and would further inhibit infection and abscess.
The types of materials which have been suggested as retrofilling materials consist of many of the standard orthograde dental filling materials, and include gutta percha, zinc oxide eugenol paste, cavity, composite resins, gold foil, glass ionomers, standard amalgams and other materials. The suitability of these materials as retrograde filling materials have been tested by their sealing ability, marginal adaption to the dentinal walls, biocompatibility and their clinical performance. To date, no material has been found which satisfies all the desired properties of a retrofilling material.
For example, amalgam has been the most commonly used retrograde filling material for many years, even though it has many disadvantages. One of its primary disadvantages is the allowance of initial leakage. When initially applied the amalgam has poor adaption to the dentinal cavity walls and allows leakage between the interface of the amalgam and dentinal walls. This leakage reduces over time due to secondary corrosion as the interface is exposed to the oral fluids. Even with secondary corrosion, the effectiveness of the seal has been questioned, and the corrosion products in the gap between the amalgam and the cavity walls may themselves be detrimental. Additional disadvantages of amalgam include (a) mercury and lead contamination; (b) non-sterility; (c) moisture sensitivity; (d) need for an undercut in the cavity preparation; (e) staining of hard and soft tissues; (f) scatter of amalgam particles; and (g) the need to apply a cavity varnish to the dentinal walls to limit initial leakage.
Because of these disadvantages of amalgam, zinc oxide eugenol ("ZOE") based cements such as Super EBA and IRM have also been used as retrograde filling materials. The disadvantages of ZOE-based cements include: (a) moisture sensitivity; (b) irritation of vital tissue; (c) solubility; (d) need for an undercut in the cavity preparation; and (e) difficulty in clinical handling of the material.
Thus, a need exists for a material to repair and reconstruct hard tissue of the anatomy, and more particularly a need exists for an improved dental filling and sealing material which can be applied in either an orthograde or retrograde cavity. It should satisfy all or most of the ideal characteristics of such filling materials. Namely, it should provide an improved seal at the surface of the tooth structure and the cavity thereby preventing the migration of bacteria and material into the cavity or periapical tissues. The filling material should also be easily applied, able to form a structurally solid filling, be biocompatible, be compatible with the presence of moisture, and easily sterilized.